Preparation of tetrafluoroethylene fine powder

ABSTRACT

Tetrafluoroethylene fine powder resins are described which have surprisingly high extrusion pressures and molecular weights which make them useful in post-paste extruded stretching operations. The resins are made by using a manganese triacetate/selected reducing agent polymerization initiator and controlling its rate of addition.

FIELD OF THE INVENTION

This invention relates to a process for preparing novel tetrafluoroethylene (TFE) fine powder resins, and particularly to such resins that have good stretch performance.

BACKGROUND OF THE INVENTION

Tetrafluoroethylene (TFE) fine powder resins are non-melt-fabricable and are commonly processed by paste extrusion wherein the powder is mixed with a lubricant and is then discharged through a paste extruder to obtain films, tubes, tapes, protective coating on wire and the like.

Such paste extruded films, tubes and tapes can be rapidly stretched in unsintered form to form a strong material that is porous to water vapor but not to liquid water. Such a material is useful in providing "breathable" fabric material for garments, tenting, separatory membranes and the like. Heretofore, the resins useful in making such paste extruded stretched films exhibited a sensitivity to lubricant loading levels and to stretch rates that necessitated careful control over the loading level used in order to ensure a good stretched product.

It is desirable to improve on these known resins by providing improved TFE fine powder resins that are not as sensitive to lubricant loading levels and which have improved stretchability. This invention is directed to a process for obtaining them.

SUMMARY OF THE INVENTION

This invention provides a process for preparing tetrafluoroethylene resins by polymerizing tetrafluoroethylene, and, optionally, a small amount of a selected copolymerizable fluorinated ethylenically unsaturated comonomer, in an aqueous medium in the presence of a substantially non-telogenic anionic surfactant present in an amount which maintains colloidal particles of polymerization product in dispersed form, said process being carried out by contacting tetrafluoroethylene and, optionally, said selected comonomer, in the presence of a redox polymerization initiator system defined as a mixture of

(i) manganese triacetate oxidizing agent, and

(ii) a substantially non-telogenic reducing agent (preferably the agent will be selected from oxalic acid, malonic acid, citric acid, ferrous iron, sodium bisulfite, sodium thiosulfate, hydroxylamine, hydrazine, or mixtures thereof),

and wherein the initiator system is added as a precharge or intermittently or continuously, and where the last addition occurs so that the reaction slows down and the end point is at least 10%, preferably 20%, longer in comparison with a reaction where initiator addition is continued to the end of the polymerization.

This process produces an aqueous dispersion of the resins. On coagulation of the dispersion, the resins are obtained. The resins have an unusual insensitivity to lubricant loading levels, have high stress relaxation times (dwell times), and can be stretched at low stretch rates, e.g. 10% per second. to make porous articles. The dispersions are useful for coating metals and fabrics.

DESCRIPTION OF THE INVENTION

The polytetrafluoroethylene obtained by the process of this invention is referred to by those skilled in the art as tetrafluoroethylene fine powder resin. The term "fine powder" has attained a special meaning in the art. It means that the resin has been prepared by the "aqueous dispersion polymerization" process. In this process sufficient dispersing agent is employed and agitation is mild in order to produce small colloidal size particles dispersed in the aqueous reaction medium. Precipitation (i.e., coagulation) of the resin particles is avoided during the polymerization.

There is another polytetrafluoroethylene material called "granular polytetrafluoroethylene" which is prepared by polymerizing tetrafluoroethylene by a process in which little or no dispersing agent is employed and agitation is carried out vigorously in order to produce a precipitated resin. This process is called "suspension polymerization".

The two polymerization procedures produce distinctly different products. The "granular" product can be molded in various forms, whereas the product produced by the aqueous dispersion method cannot be molded but must be fabricated by dispersion coating or by coagulating to obtain fine powder and then adding a lubricant to the powder for paste extrusion. In contrast, granular resin is incapable of being paste extruded.

Tetrafluoroethylene may be polymerized alone in the process of this invention to obtain a fine powder homopolymer resin of the invention. In addition, tetrafluoroethylene may be copolymerized with copolymerizable fluorinated ethylenically unsaturated comonomer provided the amount of comonomer is not sufficient to cause the resulting polymer to become melt-fabricable.

Representative copolymerizable fluorinated ethylenically unsaturated comonomers are represented by the formulas ##STR1## wherein R₁ is --R_(f), --R_(f) --X, --O--R_(f) or --O--R_(f) --X in which --R_(f) is a perfluoroalkyl radical of 1-10 carbon atoms, --R_(f) --is a linear perfluoroalkylene diradical of 1-10 carbon atoms in which the attaching valences are at each end of the linear chain, and X is H or Cl; R₂ is F, --R_(f) or --R_(f) --X; and R₃ is H or F. A dioxole may also be employed, of the formula ##STR2## where Y is ##STR3## or ##STR4## and X and X' are F or Cl and Z and Z' are each alkyl or fluorinated alkyl of 1-6 carbons.

Representative copolymerizable fluorinated ethylenically unsaturated comonomer includes hexafluoropropylene, perfluorohexene-1, perfluorononene-1, perfluoro(methyl vinyl ether), perfluoro(n-propyl vinyl ether), perfluoro(n-heptyl vinyl ether), perfluoromethyl ethylene, perfluorobutyl ethylene, ω-hydroperfluoropentene-1, 3-hydroperfluoro(propyl vinyl ether), and the like, or mixtures thereof such as a mixture of hexafluoropropylene and perfluoro(propyl vinyl ether). Preferably the comonomers are selected from perfluoro(alkyl vinyl ethers) of the formula R_(f) --O--CF═CF₂ ; or perfluoro(terminally unsaturated olefins) of the formula R_(f) --CF═CF₂ ; or perfluoroalkyl ethylenes of the formula R_(f) --CH═CH₂, wherein R_(f) is perfluoroalkyl of 1-10 carbon atoms.

By the term "non-melt-fabricable" is meant a tetrafluoroethylene polymer whose melt viscosity is so high that the polymer cannot be easily processed by melt fabrication techniques. Generally the higher the molecular weight of the polymer, the higher the melt viscosity. A melt viscosity above which tetrafluoroethylene polymers are non-melt-fabricable is 1×10⁹ poises. The melt viscosities of non-melt-fabricable polymers are so high that molecular weights are usually measured indirectly by a procedure which gives the standard specific gravity (SSG) of the resin. The SSG of the resin varies inversely with molecular weight; as the molecular weight increases, the numerical value of the SSG decreases.

The resins produced herein, in general, have the following characteristics:

(a) The primary particle size is between 0.1 and 0.5 microns,

(b) the specific surface area is greater than 5 m² /g,

(c) the standard specific gravity is less than 2.190,

(d) the rheometric pressure (sometimes referred to as extrusion pressure) is at least 250 kg/cm², at reduction ratio of 400:1,

(e) the uniformity of stretch is at least 75% throughout a lubricant loading range of 4 weight percent which 4 weight percent range is within a lubricant loading level range between 10 and 20 weight percent at a stretch rate of 100%/second.

(f) the uniformity of stretch is at least 75% throughout a stretch rate of between 10 and 100%/second at a lubricant loading level of 17%.

(g) the stress relaxation time is at least 400 seconds measured at 393° C.

In a preferred embodiment, the uniformity of stretch is at least 75% throughout a lubricant loading range of between 17 and 20 wt. % at a stretch rate of 100%/second; and also is at least 75% throughout a stretch rate of between 22% and 100%/second at a lubricant loading of 17 wt. %.

In the process of this invention, tetrafluoroethylene monomer, optionally along with ethylenically unsaturated comonomer, is admixed or contacted with an aqueous medium containing dispersing agent and polymerization initiator. The polymerization temperature and pressure are not critical provided the reaction profile recited above is used. Temperatures useful to decompose the Mn⁺³ initiator system are desirable in order to obtain high molecular weight near the surface of the resin particles formed. Ideally the temperature will be between 50°-125° C.; and preferably between 65°-100° C. A practical, but noncritical, pressure can be between 15-40 kg/cm², and preferably 25-40 kg/cm². The polymerization is ordinarily carried out in a gently stirred autoclave.

The dispersing agents used are anionic and are substantially nontelogenic. Commonly employed dispersing agents are fluorinated carboxylates containing 7-20 carbon atoms, such as ammonium polyfluorocarboxylates. The amount of dispersing agent present will be sufficient to stabilize the colloidal dispersion. It may be ordinarily between about 1000 ppm and about 5000 ppm based on weight of water employed in the aqueous dispersion. The dispersing agent may be added prior to initiation of polymerization or may be added in increments as described in Punderson U.S. Pat. No. 3,391,099.

If desired, a paraffin wax (i.e., a saturated hydrocarbon having more than 12 carbon atoms) that is liquid at the polymerization temperature may be employed as described in Bankoff U.S. Pat. No. 2,612,484. Usually, the wax is employed in an amount between 0.1%-12% by weight of water in the aqueous dispersion.

Polymerization is effected by mixing the foregoing described ingredients under the conditions specified above. Mixing is ordinarily carried out by mildly agitating the aqueous polymerization mixture. Agitation is controlled to aid in preventing premature coagulation of resin particles produced in the polymerization. Polymerization is ordinarily conducted until the solids level (i.e., polymer content) of the aqueous mixture is between about 15 and 60 percent by weight of the mixture.

By the term "substantially non-telogenic" used in the definition of the dispersing agent is meant that the polymer produced has an SSG (standard specific gravity) no more than 0.010 higher than the SSG of a polymer produced under the same conditions but using ammonium perfluorooctanoate as the dispersing agent. (SSG is a measure of the molecular weight of the polymer). Similarly, with respect to the reducing agent, the term non-telogenic means that the polymer produced has an SSG no more than 0.010 higher than the SSG of a polymer produced under the same conditions but using oxalic acid as the reducing agent.

The initiator may be added to the polymerization vessel optionally as a precharge, and/or in increments, or continuously during the polymerization, provided that the selected reducing agent, preferably oxalic acid, is present to form a redox couple with the manganese triacetate.

The amount of manganese triacetate used will be between 5-500 ppm, preferably 25-100 ppm, based on aqueous charge, depending on the molecular weight desired in the polymer. The amount of reducing agent used will also be 5-500 ppm, preferably 25-100 ppm, based on aqueous charge.

The reaction is generally carried out in acidic medium. Succinic acid is a common acid and is preferred because it also prevents coagulation. Buffers may be used to control the pH.

On completion of polymerization, the dispersed polymer particles can be coagulated by high speed agitation. The particles can then be collected and dried.

Non-melt fabricable tetrafluoroethylene fine powder resins produced by the process of this invention exhibit excellent stretch performance at elevated temperatures, e.g. 300° C., to result in a stretched material that is strong and breathable but impervious to liquid water. The resins are of high molecular weight, having an SSG of less than 2.190. They have a high rheometer pressure which is at least 250 kg/cm². They have a primary particle size between 0.1 and 0.5 micron. By "primary" is meant the size of the colloidal resin particles measured prior to coagulation. The resins also have a specific surface area greater than 5 m² /g.

In addition, the resins of this invention have several unusual stretch features. First, the resins can be paste extruded over a wide range of amount of lubricant additive present. Normally fine powder resins are sensitive to the amount of lubricant present during paste extrusion and as the amount is varied, the properties of the paste extruded product will vary widely. Uniquely, with the resins of this invention, the amount of lubricant can vary widely, e.g. from at least over a loading range of 4% within a total range of 10 wt % to 20 wt %, with no significant loss of stretch uniformity and smoothness of surface at a stretch rate of 100%/second. This is an insensitivity to organic lubricant loading levels that is not ordinarily seen in other fine powder resins. Suitable organic lubricants include hexane, heptane, naphtha, toluene, xylene, and kerosene products such as Isopar K and E. In general these lubricants will have a viscosity of at least 0.3 centipoise at 25° C. and will be liquid under extrusion condition. Preferably they will contain paraffins, naphthenes and aromatics and small amounts of olefin.

In addition, the resins of this invention exhibit an unusual insensitivity to stretch rate. Most fine powder resins exhibit inferior stretch performance as stretch rates are lowered. But surprisingly when the stretch rate of a resin of this invention was varied between 10% per second and 100% per second, the stretched product exhibited no significant change in stretch uniformity or surface smoothness at a lubricant loading level of 17 wt %. Specifically, the uniformity of stretch was at least 75%. This means that an ink mark made at the center of a paste extruded beading before stretching did not move more than 25% from the center of the stretched product.

In addition, the stress relaxation times of the resins of this invention are significantly greater than for most other fine powder resins.

The resins of this invention are useful in any of the paste extrusion applications that known tetrafluoroethylene fine powder resins are useful.

TEST PROCEDURES

(1) Raw Dispersion (Primary) Particle Size (Avg)

RDPS was determined from the absorbance (scattering) of a dilute aqueous sample at 546 millimicrons using a Beckman DU spectrophotometer and is based on the principle that the turbidity of the dispersion increases with increasing particle size, as shown in U.S. Pat. No. 4,036,802.

(2) Standard Specific Gravity (SSG)

SSG was measured by water displacement of a standard molded test specimen in accordance with ASTM D1457-69. The standard molded part was formed by preforming 12.0 g of the powder in a 2.86 cm diameter die at a pressure of 352 kg/cm², followed by the sintering cycle of the preform of heating from 300° C. to 380° C. at 2° C./min, holding at 380° C. for 30 min, cooling to 295° C. at 1° C./min and holding at this temperature for 25 minutes, after which the specimen is cooled to 23° C. and tested for specific gravity.

(3) Rheometer Pressure

Rheometer pressure was measured in accordance with ASTM D1457-81A, section 12.8, except that the resin was not sieved before mixing with the kerosene lubricant and the preform was made in a 26 mm diameter extension tube at 300 psi. A reduction ratio of 400:1 was used.

(4) Specific Surface Area (SSA)

SSA was measured by a "Quantasorb" surface area analyzer sold by Quanta Chrome Corp. The analyzer was calibrated by the B.E.T. method.

(5) Stretch Test

a. Preparation of Test Specimen

A sample of the resin was screened through a 2000 microns sieve. One hundred grams of this resin was admixed with the desired amount of Isopar K lubricant at room temperature by shaking in a glass jar of 6 cm inside diameter and rolling for 4 min. at 64 rpm. It was then preformed at room temperature in a tube 26 mm diameter×23 cm long at 400 psi. The preform was then paste extruded at room temperature through an orifice 2.4 mm in diameter into a uniform beading. Land length of the orifice was 5 mm. The extrusion speed was 84 cm/min. The angle of die was 30° . The beading was dried at 190° C. for 20 minutes.

b. Stretch Test

A beading of resin was cut and clamped at each end leaving a space of 50 mm between clamps, and heated to 300°C. in a circulating air oven. The clamps were then moved apart at the desired rate to the desired length. The stretched specimen was examined for uniformity of stretch, even appearance and surface roughness. The % uniformity was calculated as follows: ##EQU1## (6) Stress Relaxation Time

The specimen for the relaxation time measurement was made by stretching a beading, as in Stretch Test, at 1000% per second and 2400% total stretch. Stress relaxation time is the time it takes for this specimen to break when heated at 393° C. in the extended condition. For a short period of time when the specimen is placed into the oven, the temperature drops somewhat, e.g., to 375° C. and it takes about one minute for the oven to return to 393° C. Stress relaxation time is the time starting from placement of the test specimen into the oven.

EXAMPLES Example 1

A 36-liter polykettle was charged with 19.1 kg of demineralized water, 600 g paraffin wax, 16 g ammonium perfluorooctanoate (C-8) dispersant, 0.15 g zinc chloride, 1.0 g oxalic acid, and 3 g succinic acid. The contents of the polykettle were heated to 70° C., evacuated, and TFE purged. The contents of the polykettle were agitated at 46 RPM. The temperature was increased to 80° C. TFE was then added until the pressure was 2.75×10⁶ Pa. A fresh solution of 3.2 g manganese triacetate, Mn (III) acetate, dissolved in 1000 ml of water-acetic acid mixture (1:1 by volume) was injected at the rate of 6.5 ml per minute until 8.2 kg TFE had reacted. Polymerization began after 4 minutes from the start of the initiator injection, as evidenced by a drop in pressure. TFE was added to maintain the pressure at 2.75×10⁶ Pa. After 0.9 kg TFE had reacted, a solution of 42 g ammonium perfluorooctanoate in 1000 ml water was pumped in at 50 ml/min. The total manganese salt added was 0.768 g. No manganese triacetate was added after 67% of the TFE had been polymerized. The reaction was 58% longer than if the Mn salt addition had continued until the end. After 12.3 kg TFE had reacted, the feed was stopped and the polykettle was vented, evacuated, and purged with N₂. The contents were cooled and discharged from the polykettle. The supernatant wax was removed. The dispersion was diluted to 15% solids and coagulated in the presence of ammonium carbonate under high agitation conditions. The coagulated fine powder was separated and dried at 150°-160° C. for three days.

The polymer properties are given in Tables I and II. The stretch performance was excellent. The total reaction time from TFE pressure up to feed off was 79 minutes compared to 123 minutes for the Comparative Run A (described below).

Comparative Run A

The polykettle described in Example 1 was charged with 20 kg demineralized water, 600 g paraffin wax, 13 g APFC (C-8) surfactant, and 10 g succinic acid. After a TFE pressure of 2.75×10⁶ Pa was obtained, 120 ml ammonium persulfate (APS) solution (1.0 g/l) was added at 100 ml/min., at 75° C. After 0.9 kg TFE had reacted, pumped in a solution of 45 g additional C-8 in 1000 ml water at 50 ml/min. The temperature was maintained at 75° C. After 14.1 kg TFE had reacted, the feed was stopped and the polykettle was allowed to react down to 1.72×10⁶ Pa before venting. Fine powder was obtained after processing as in Example 1.

The polymer properties are given in Tables I and II. The stretch performance was good but the total reaction time was 123 minutes.

This Example shows that the use of a commonly used initiator, such as APS, causes substantially longer reaction time than the manganese salt for satisfactory stretch performance.

Comparative Run B

Example 1 was repeated, except for continued injection of the manganese triacetate solution until the end of the polymerization. The total manganese salt added was 1.267 g. The reaction time was 61 minutes. The stretch speciment broke during stretching.

The polymer properties are given in Tables I and II. This Example shows that the polymer stretch performance is unsatisfactory when the initiator is injected until the end of the polymeriztion, causing lower molecular weight for the particle exterior.

Comparative Run C

Example 1 was repeated, except that oxalic acid, succinic acid, and ZnCl₂ were not used. There was no reaction.

This Example shows that in the absence of a reducing agent, polymerization of TFE does not take place.

                  TABLE I                                                          ______________________________________                                                          Comparative                                                              Example 1                                                                              Run A    Run B   Run C                                      ______________________________________                                         Total Reaction                                                                              79        123      61    No                                       Time, min (1)                         Reaction                                 RDPS, Micron 0.245     0.252    0.217 --                                       SSG          2.1737    2.166    2.195 --                                       Specific Surface                                                                            11.6      11.1     14.1  --                                       Area, m.sup.2 /g                                                               Rheometer Pressure,                                                                         378       390      293   --                                       kg/cm.sup.2 (RR = 400:1)                                                       Stress Relaxation                                                                           570       555      255   --                                       Time, sec                                                                      ______________________________________                                          (1) From TFE pressure up to TFE feed off.                                

                  TABLE II                                                         ______________________________________                                         In Table II, the numbers show the percent                                      uniformity of stretch at the conditions described and                          the letter indicates surface smoothness at the                                 conditions shown.                                                                                   Comparative                                                            Example 1 Run A   Run B                                           ______________________________________                                         Uniformity of Stretch                                                                         90A         97A     D                                           at Lubricant Loading                                                           of 17% and Stretch                                                             Rate of 100% per second                                                        (1)                                                                            Uniformity of Stretch                                                                         78C          D      --                                          at Lubricant Loading                                                           of 23% and Stretch                                                             Rate of 100% per second.sup.1                                                  (1)                                                                            Uniformity of Stretch                                                                         83A         78B     --                                          at Lubricant Loading                                                           of 17% and Stretch                                                             Rate of 22% per second                                                         (2)                                                                            Uniformity of Stretch                                                                         89A         68C     --                                          at Lubricant Loading                                                           of 10% and Stretch                                                             Rate of 100% per second                                                        (1)                                                                            ______________________________________                                          (1) 1500% of total stretch                                                     (2) 1000% of total stretch                                                     A Smooth even surface                                                          B Slightly uneven surface                                                      C Uneven surface                                                               D Specimen severed (broke) during stretch test                                 .sup.1 Here the loading of lubricant is too high to produce good results. 

We claim:
 1. A process for polymerizing tetrafluoroethylene in an aqueous medium in the presence of a substantially non-telogenic anionic surfactant present in an amount which maintains colloidal particles of polymerization product in dispersed form, said process being carried out by contacting tetrafluoroethylene in said aqueous medium in the presence of a redox polymerization initiator system defined as a mixture of(i) manganese triacetate oxidizing agent, and (ii) a substantially non-telogenic reducing agent and wherein the redox initiator components are added such that the last portion of the polymerization is slowed down by stopping addition of such components so that the end point of the reaction is at least 10% longer than if addition of such component is continued to the end of the polymerization.
 2. The process of claim 1 wherein tetrafluoroethylene is the only monomer present.
 3. The process of claim 1 wherein at least one comonomer is present which is of the formula ##STR5## wherein R₁ is --R_(f), --R_(f) --X', --O--R_(f) or --O--R_(f) --X' in which --R_(f) is a perfluoroalkyl radical of 1-10 carbon atoms, --R_(f) --is a linear perfluoroalkylenediradical of 1-10 carbon atoms in which the attacking valences are at each end of the linear chain, and X' is H or Cl; R₂ is F, --R_(f) or --R_(f) --X; and R₃ is H or F; or of the formula ##STR6## where Y is ##STR7## or ##STR8## X and X' are F or Cl and Z and Z' are alkyl or fluorinated alkyl of 1-6 carbons.
 4. The process of claim 1 wherein the reducing agent is selected from oxalic acid, malonic acid, citric acid, ferrous iron, sodium bisulfite, sodium thiosulfate, hydroxyl amine, hydrazine, or mixtures thereof.
 5. The process of claim 1 wherein the reducing agent is oxalic acid. 